<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>classname</title>
    <style>
        .na{
            width:150px;
            height:150px;
            background:url('0.jpg');
            background-color: red;
        }
    </style>
</head>
<body>
<div class="na"></div>
<div class="na"></div>
<script>
    var a=document.getElementsByClassName("na");
    console.log(a);
    console.log(a instanceof Object);
    //类数组
</script>
</body>
</html>